1. Field of the Disclosure
The present disclosure concerns the making of high performance concrete obtainable without significant quantities of added materials with latent hydraulic activity, in particular silica fume.
2. Discussion of the Background Art
Before describing in detail the present disclosure it would be proper to define the meaning of some terms and refer to the state of the art gathered by reading technical and patent bibliography on the subject.
Currently the expression “high performance” refers to a rather large range of usable characteristics of cement mixtures and in particular the rheological and mechanical performances.
With regards to the rheological performance, a high performance concrete is generally required to have at least a thixotropical or, preferably, a self-compacting behaviour.
With regards to the mechanical performance, the expression “high performance” refers to resistance to a large range of mechanical stress. According to the guide-lines of the Italian Ministry of Public Works inistry concretes can be classified as high performance concrete (HPC) if they are characterised by a water/cement ratio of <0.45 and a characteristic cubic resistance at 28 days of between 55 MPa and 75 MPa, or as High resistance concrete (AR) if characterized by a water/cement ratio of <0.35 and a characteristic cubic resistance of between 75 MPa and 115 MPa. In this disclosure, the term “high performance” includes the HPC concrete and AR concrete.
It is well known that, in order to obtain high performance concrete, it is necessary to adopt a reduced water/binder ratio (by binder we mean Portland cement and any other suitable material) and a dosage of binder generally higher than that required for a concrete with normal resistance.
The present European standards for concrete (EN 206-1) allow the addition to the concrete of materials finely subdivided in order to obtain or improve some proprieties. The EN 206-1 takes into account two types of additions:                The additions which are nearly inert (type I), from among which it is possible to mention limestone;        The additions with a pozzolanic activity or a latent hydraulic activity (type II) are the fly ash in conformity with standard EN 450 and silica fume in conformity with standard EN 13263.        
It is well known that high performance concrete can be produced starting from a great variety of cement systems such as:                pure Portland cement;        Portland cement and fly ash;        Portland cement and silica fume;        Portland cement, slag and silica fume;        
According to data found in literature, nearly all high performance concrete contain silica fume.
There have been numerous studies on the effects of silica fume on the hydration reactions of cement systems. It is well known that the action of silica fume manifests itself as a pozzolanic addition and as a filler. The action of silica fume as a granular filling material amongst the cement particles is explained by virtue of its extremely reduced dimensions (between 30 and a 100 times smaller that those of Portland cement).
As a counterpoint to the advantages of using such additional material, one must point out that the addition of silica fume, in some cases, seems to cause an increase in the shrinkage in the plastic phase and can bring about noticeable phenomena of micro-cracks/SP 186-39 page 671 (E& FN SPON—Modern Concrete Technology 5-4 page 191 Ed. 1998) and (S. Rols et al. “Influence of Ultra Fine Particle Type on Properties of Very—High strength Concrete ACI SP 186 page 671-685—Proceedings of Second CANMET/ACI International Conference, RS, Brazil, 1999). There have also been indications of an unexpected resistance downgrading over the long period that can bring about a deterioration of the material. One must finally point out that the use of silica fume is particularly expensive.
With regards to the limestone filler, it has for a long time been considered as inert. Although it cannot be considered as a pozzolanic addition material, many studies have shown that it has a significant reactivity in the medium and long term. S. Sprung, E. Siebel “ . . . ” Zement Kalk Gibs 1991, N. 1, page. 1-11 hypotheses that, in addition to its main role as matrix filler, the limestone filler could bring about a certain chemical reaction with aluminates to produce the formation of calcium aluminates. Ramachandran et al. (Ramachandran et al. In “Durability of Buildings Materials”, 4 1986) have observed that the addition of CaCO3 to C3S (silicate tricalcium) accelerates the hydration. Furthermore, they have found that the hydration of the cement is accelerated by the limestone filler effect. In the cement mixture, CaCO3 would produce a calcium aluminate that would be incorporated in the phases C3S and C3A (aluminous tricalcium) during the hydration. S. P. JIANG et al. (“Effect of fillers (Fine particles) on the Kinetics of Cement Hydration 3rd Beijing International Symposium on Cement and Concrete, 1993, 3) have furthermore shown that the formation of calcium aluminate is beneficial because it would increase the resistance and accelerate the hydration process. According to Jiang et al. (S. P. Jiang et al. 9th International Congress of Cem. Chem. New Delhi, 1992) the limestone filler would act on the hydration kinetics of cement. According to such authors, the acceleration of the hydration can be attributed, rather than to what has been described above, to a multiplication effect of the inter-particles contacts and to the nature of such contacts on the surface of the limestone filler.
With regards to the high performance, from a rheologic point of view, it is well known that the self-compacting concrete is becoming important.
The self-compacting concrete (SCC) is a special concrete capable of flowing, by the sole reason of its own weight, in the formworks and to flow around obstacles, such as the reinforcement bars, without stopping and giving place to separation of its constituents. Its rheological characteristics must be preserved until the setting and hardening process starts.
According to the AFGC (Association Francaise de Genie Civil) the auto-compacting concrete must satisfy the following requisites when fresh:    a) The values of expansion of Slump Flows (slump cone) must fall generally in a field between 60 and 75 cm (with no visible segregation at the end of the test—that is to say no lime wash aureole along the external perimeter and no concentration in the centre);    b) the filling ratio of the equipment denominated L-Box must be higher than 80%;    c) the concrete must not show segregation and must present a limited bleeding.
A patent application (MI2001A002480) has been filed regarding concrete having a 28 days mechanical resistance higher or equal to 110 MPa comprising cement in conformity with the European standard 197-1, limestone aggregates and additives, characterized by the fact that any additions with latent hydraulic activity are lower than 5% with respect to the weight of the cement.
Concretes according to the Italian patent application MI2001A002480 were characterized by the following characteristics:    1) the absence of significant quantities of additions of type II (for example silica fume)    2) 28 days mechanical resistance higher than 110 MPa and/or a development of the mechanical compression resistance so as to guarantee the values shown in the following table
TABLE Iperformance requirements in terms of themechanical resistance development.Expiry [days]1228Rc [MPa]≧50≧80≧110    3) cement quantity between 25% and 50% in weight with regards to the total weight of the solid mixture.    4) limestone aggregates having a D. Max between 2 and 12 mm.
These characteristics were reached by using a granulometric curve of the solid mixture which is quite different from those previously known in art.
Although they present remarkable advantages, concretes thus obtained have proved to have a limited industrial desirability due to an excessively high cement dosage that, apart from being responsible for temperature rises from which self-tensioning can arise, can provoke relevant phenomena of autogenous shrinkage and an undesirable increase in costs.
It is therefore desirable to have a concrete that maintains the performance of the above mentioned patent (MI200A002480) and still maintain significantly lower the quantities of cement. More specifically, for many production applications, it would be useful to keep the resistances for short to medium term (1, 3, 7 days): this is reflected in a rapid consolidation of the mass, a quick release from the caissons and a quick production procedure; furthermore it is not always indispensable to keep the resistance values at 28 days higher than 110 MPa, as this value is much higher than the average limits for AR concrete. On the other hand a simple reduction of the cement content (with regards to the water of the mixture and/or the aggregates present) using the other parameters reported in the quoted patent application does not bring the results wished for in terms of rheology and thus not making the concrete suitable for structural use due to an excessive plasticity. The need for concrete that presents high resistance remains therefore unsolved and there is no evident solution, especially in the short and medium term, even without additions with pozzolanic activity and using a moderate quantity of cement, so as to avoid undesirable phenomena of shrinkage and reduce the total cost of the product.